New research claims to settle the controversial and long-running debate over whether domestic sheep can infect bighorn sheep with the bacteria that cause fatal respiratory pneumonia.

A research team led by Dr. Sri Srikumaran at the Washington State University College of Veterinary Medicine tagged Mannheimia haemolytica isolated from domestic sheep to show that domestic sheep could transmit the deadly bacteria to bighorn sheep when they were in contact with each other.

The four bighorn sheep in the study all died from pneumonia traced back to bacteria originating in the domestic sheep.

The bighorns showed no signs of illness while they were kept 10 meters (32.8 feet) away from the domestics during the first phase of the study. It was only after the animals were allowed to commingle that the bighorns became visibly ill and eventually died.

"There can't be any other source from which the bighorn sheep could have obtained these organisms other than the domestic sheep," explained Dr. Srikumaran, who holds the Rocky Crate-Wild Sheep Foundation endowed chair at WSU.

Kevin Hurley, the bighorn sheep coordinator for the Wyoming Game and Fish Department, called the findings compelling and unsurprising. "I don't know how anyone can disagree with the conclusions," he said.

Dr. Srikumaran's research, which appeared in the July issue of the Journal of Wildlife Diseases, follows on the heels of an unprecedented high number of pneumonia outbreaks among bighorn sheep in several Western states (see JAVMA, May 1, 2010, page 936).

Bighorn sheep are particularly susceptible to pneumonia. It's long been suspected that a leading cause of infection is contact with domestic sheep, which harbor the bacteria yet aren't sickened by it. Wool farmers have mostly rejected such theories, however, and worry that, if proved, they'll lose access to publicly owned grazing lands that are home to the bighorns.

Dr. Peregrine Wolff, the Nevada state wildlife veterinarian, says Dr. Srikumaran's research "proves unequivocally what the sheep have been telling us for years": When bighorn sheep have contact with domestic sheep, the results can be catastrophic.

"Why many have refused to accept that animals of the same genus that have evolved on different continents or environments can have drastically different responses to the same pathogens has always puzzled me," she added.

Previous studies have shown that bighorn sheep die off when they are commingled with domestic sheep. What distinguishes Dr. Srikumaran's research is compelling evidence that bacteria that killed the bighorns in his study originated in domestics.

A plasmid containing a green fluorescent protein gene along with an antibiotic-resistance gene was used to tag M haemolytica isolated from four domestic sheep. M haemolytica tagged with both markers were then returned to the test domestic sheep.

The bighorn and domestic sheep were kept apart by a span of 10 meters for one month. Next, researchers allowed the animals to have minimal contact through a chain-link fence for two months. In that time, tests showed three of the bighorns had acquired the tagged organisms, according to Dr. Srikumaran.

Toward the end of the two-month period, one of the bighorns was coughing, but the animal and its companions were otherwise fine.

Finally, the sheep were allowed to mix. "Two days after they commingled, one of the bighorns died. Five days later, another two animals died, and on the ninth day, we had to euthanize the last remaining bighorn because it was very sick with pneumonia," Dr. Srikumaran said.

The tagged organisms were recovered from the bighorn sheep at necropsy, and additional testing showed that they were indeed the same pathogens that originated in the domestic sheep. "How do we know these are the same organisms that came from domestic sheep?" Dr. Srikumaran asked. "Because they're genetically modified organisms not found in nature."

Dr. Srikumaran reasons the bighorns were infected when they were allowed fence-line contact with the domestics. "I feel that if we'd allowed them to stay with the fence-line contact beyond two months, (the bighorns) would've died," he said.

For Dr. Srikumaran, the study provides "irrefutable" evidence domestic sheep can infect bighorns with the fatal respiratory bacteria. The long-term goal of his laboratory is developing control measures against pneumonia infections.

Dr. Wolff, for whom the pneumonia-related bighorn die-off in Nevada has been a pressing concern, hopes this latest study will result in renewed support for best management practices aimed at preventing nose-to-nose contact between domestic and bighorn sheep.